Flexographic printing is widely used in the production of newspapers and in the decorative printing of packaging media. Numerous photosensitive printing plate formulations have been developed to meet the demand for fast, inexpensive processing and long press runs.
Photosensitive printing elements generally comprise a support layer, one or more photosensitive layers, an optional slip film release layer, and an optional protective cover sheet. The protective cover sheet is formed from plastic or any other removable material that can protect the plate or photocurable element from damage until it is ready for use. The slip film release layer may be disposed between the protective cover sheet and the photocurable layer(s) to protect the plate from contamination, increase ease of handling, and act as an ink-accepting layer. After exposure and development, the photopolymer flexographic printing plate consists of various image elements supported by a floor layer and anchored to a backing substrate.
Flexographic printing plates desirably work under a wide range of conditions. For example, they should be able to impart their relief image to a wide range of substrates, including cardboard, coated paper, newspaper, calendared paper, and polymeric films such as polypropylene. Importantly, the image should be transferred quickly and with fidelity, for as many prints as the printer desires to make.
Flexographic printing elements can be manufactured in various ways including with sheet polymers and by the processing of liquid photopolymer resins. Flexographic printing elements made from liquid photopolymer resins have the advantage that the uncured resin can be reclaimed from the non-image areas of the printing elements and used to make additional printing plates. Liquid photopolymer resins have a further advantage as compared to sheet polymer in terms of flexibility, which enables the production of any required plate gauge simply by changing the machine settings. The plates are typically formed by placing a layer of liquid photopolymerizable resin on a glass plate but separated from the glass plate by a substrate and/or a coverfilm. Actinic light, such as UV light, is directed against the resin layer through a negative. The result is that the liquid resin is selectively cross-linked and cured to form a printing image surface that minors the image on the negative. Upon exposure to actinic radiation, the liquid photopolymer resin polymerizes and changes from a liquid state to a solid state to form the raised relief image. After the process is complete, non-crosslinked liquid resin can be recovered (i.e., reclaimed) from the printing plates and recycled in the process to make additional plates.
Residual traces of liquid resin remaining in the regions of the resin that were protected from actinic radiation by the opaque regions of the transparency are then washed away using a developer solution. The cured regions of the printing element are insoluble in the developer solution, and so after development, a relief image formed of cured photopolymerizable resin is obtained. The cured resin is likewise insoluble in certain inks, and thus may be used in flexographic printing. The liquid photopolymerizable resin may also be exposed to actinic radiation from both sides of the resin layer,
Various processes have been developed for producing printing plates from liquid photopolymer resins as described, for example, in U.S. Pat. No. 5,213,949 to Kojima et al., U.S. Pat. No. 5,813,342 to Strong et al., U.S. Pat. Pub. No. 2008/0107908 to Long et al., and in U.S. Pat. No. 3,597,080 to Gush, the subject matter of each of which is herein incorporated by reference in its entirety.
Typical steps in the liquid platemaking process include:
(1) casting and exposure;
(2) reclamation;
(3) washout;
(4) post exposure;
(5) drying; and
(6) detackification.
In the casting and exposure step, a photographic negative is placed on a glass platen and a coverfilm is placed on the negative in an exposure unit. All of the air is then removed by vacuum so that any wrinkling of the negative or coverfilm can be eliminated. Thereafter, a layer of liquid photopolymer and a backing sheet (i.e., a thin layer of polyester or polyethylene terephthalate) are applied on top of the coverfilm and negative. The backing sheet may be coated on one side to bond with the liquid photopolymer and to serve as the back of the plate after exposure. Then upper and/or lower sources of actinic radiation (i.e., UV lights) are used to expose the photopolymer to actinic radiation to crosslink and cure the liquid photopolymer layer in the areas not covered by the negative. The top sources of actinic radiation are used to create the floor layer of the printing plate (i.e., back exposure) while the bottom sources of actinic radiation are used to face expose the photopolymer to actinic radiation through the negative to create the relief image. Plate gauge may be set by positioning a top exposure glass at a desired distance from a bottom exposure glass after dispensing liquid photopolymer on the protected bottom exposure glass.
After the exposure is complete, the printing plate is removed from the exposure unit and the photopolymer that was not exposed to actinic radiation (i.e., the photopolymer covered by the negative) is reclaimed for further use. In liquid platemaking, resin recovery is an important factor relating to the production of photopolymerizable resin printing plates because the resins used to produce the plates are relatively expensive. In all areas not exposed to UV radiation, the resin remains liquid after exposure and can then be reclaimed. In a typical process, the uncured resin is physically removed from the plate in a process step so that the uncured resin can be reused in making additional plates. This “reclamation” step typically involves squeegeing, vacuuming or otherwise removing liquid photopolymer remaining on the surface of the printing plate. The coversheet may then be stripped from the plate. This reclamation step not only saves material costs of the photopolymer resin but also reduces the use and cost of developing chemistry and makes a lighter plate that is safer and easier to handle.
Any residual traces of liquid resin remaining after the reclamation step may then be removed by nozzle washing or brush washing using a wash-out solution to obtain a washed-out plate, leaving behind the cured relief image. Typically, the plate is placed into a washout unit wherein an aqueous solution comprising soap and/or detergent is used to wash away any residual unexposed photopolymer. The plate is then rinsed with water to remove any residual solution.
After the washout step has been completed, the printing plate is subjected to various post exposure and detackification steps. Post exposure may involve submerging the plate in a water and salt solution and performing an additional exposure of the printing plate to actinic radiation (UV light) to fully cure the printing plate and to increase plate strength. The printing plate may then be rinsed and dried by blowing hot air onto the plate, by using an infrared heater or by placing the printing plate into a post exposure oven.
If used, the detackification step may involve the use of a germicidal unit (light finisher) to ensure a totally tack-free plate surface. This step is not require for all plates, as certain resins may be tack-free and thus printing press ready without the need for the detackification step.
As discussed above, because of the material costs of the liquid photopolymers, it is desirable to reclaim as much of the liquid photopolymer as possible in the reclamation step. Therefore, it would be desirable to optimize the amount of liquid photopolymer that may be reclaimed.
The present invention relates to improvements to the reclamation step of the liquid platemaking process to more efficiently and accurately produce relief image printing elements from liquid photopolymer resins.